Generic radar devices and methods are used for short-range sensors in motor vehicles, for example. They are used, e.g., to prevent accidents and to detect objects in a dead angle of a motor vehicle.
FIG. 1 shows schematically the basic design of a conventional radar device. A carrier frequency fT is generated by a local oscillator (LO) 110. This carrier frequency is divided by a power divider 116 between a transmission branch and a reception branch. In addition to carrier frequency fT, a pulse repetition rate fPW is supplied by a pulse generator 112 for modulation of the carrier frequency. In the transmission branch, this modulation is accomplished with a switch 120 to which the carrier frequency is applied and which is switched at the pulse repetition rate. The signal thus generated is emitted by sending aerial 136. Modulation also takes place in the reception branch. However, for the purpose of this modulation, the pulses of the pulse repetition rate are delayed by a delay device 118. Modulation of carrier frequency fT is performed with these delayed pulses by actuation of switch 122 to which the carrier frequency is also applied. In this way, a reference signal SR is made available in the reception branch. This reference signal is mixed in a mixer 124 with a reception signal received via receiving aerial 134. The output signal of mixer 124 is sent to an integration means 126, e.g., a low-pass filter and an amplifier. The signal generated in this way is sent to a signal analyzer and controller 138, preferably after analog-digital conversion. Signal analyzer and controller 138 then determines the delay of delay device 118, this delay being varied between a value Δtmin and Δtmax. For example, the delay may be varied by a microcontroller or a digital signal processor. It is also possible to use special hardware for this purpose. If the transit time of the radar pulses, which usually corresponds to twice the transit time between the target and the aerial, matches the delay, the amplitude of the output signal of mixer 124 is at its maximum. Thus, a correlation receiver is available so that it is possible that the delay set by controller 138 may be used to deduce the target distance and the radial velocity between the target and the aerial. FIG. 1 shows as an example only the formation of the in-phase (I) signal. The quadrature (Q) signal is formed in a similar manner by mixing with the carrier frequency shifted in quadrature.
It is generally desirable to suppress interference signals of a wide variety of causes. There have already been proposals for utilizing additional modulation of the microwave signal to separate signal components reflected on targets from interference signals. In particular, interference due to other non-coded transmitters, e.g., radio transmitters, and/or noise is suppressed by such methods.
However, ambiguities may occur in the determination of target distances outside of the unambiguous range of a pulsed radar device. The ambiguity range at target distances r is:c/(2fPW)≦r≦Rmax where                fPW: pulse repetition rate        c: velocity of light        Rmax: radar range.        
The following target distances are measured in the ambiguity range:{circumflex over (r)}=r−nc/(2fPW)where n=1, 2, 3, . . . and {circumflex over (r)}≧0.
In addition, there is interference due to multiple pulse radar sensors operating simultaneously when the sensors are operating within the range of another particular sensor. There may be interference, i.e., for detection of apparent targets. A measured apparent signal delay and/or the corresponding distance from the apparent target will depend on the position between the transmission point in time and the reception point in time of the particular sensors. If one considers sensors installed in different vehicles, for example, it may occur that the sending aerials and receiving aerials of the different sensors are opposite one another. In this case, the interference influence between individual sensors is usually no longer negligible.